Air flow control



A ril 7, 1970 w. c. EDWARDS AIR FLOW CONTROL Filed Oct. 21, 1965 L ZO BTiL FIG. I.

FIG. 2.

INVENTOR WILLIAM C. EDWARDS ATTORN EYS United States Patent 3,504,663AIR FLOW CONTROL William C. Edwards, Plandome Manor, N.Y., assignor toSmith Kline & French Laboratories, Philadelphia, Pa., a corporation ofPennsylvania Filed Oct. 21, 1965, Ser. No. 500,431 Int. Cl. A61b /02U.S. Cl. 128-2.05 5 Claims ABSTRACT OF THE DISCLOSURE This inventionrelates to air flow controllers, and particularly to controllers of atype suitable for bleeding air or other gases from a chamber at aconstant rate so that a constant rate of decrease of pressure isproduced therein. The invention is particularly applicable to air bleeddevices forming part of a blood pressure measuring apparatus, and forpurposes of illustration will be described with reference to such anapparatus.

In the measurement of blood pressure, where an infiatable cuff is usedto constrict an artery in the arm, blood pressure is conventionallydetermined by inflating the cuif, and by decreasing the pressure in thecuff slowly, and noting the pressure at which characteristic Korotkoifsounds commence and by noting the pressure at which the sounds cease orbecome muffled. There is international agreement amongst medicalauthorities that it is desirable to reduce the pressure in the cuif at aconstant rate.

In conventional air bleed devices used both with the manual andautomatic blood pressure measuring apparatuses, air is released througha simple or manually adjustable orifice. Pressure decreases more andmore slowly as more air is bled out, and, in fact, the pressure followsan exponential function with time.

An object of the present invention, therefore, is to provide an airbleed device which produces a constant rate of pressure drop in an airchamber over a wide range of pressures.

A further difficulty is encountered in air bleed devices involving smallorifices in the matter of clogging by minute particles of lint or dust.Such clogging is particularly prevalent in devices involving taperedneedle valves provided for the purpose of adjusting flow rate.Accordingly, a further object of the present invention is to provide anair bleed device which is self-cleaning.

Other objects Will become apparent from the following description whenread in conjunction with the accompanying drawings in which:

FIGURE 1 is a vertical section showing an air bleed device in accordancewith the present invention; and

FIGURE 2 is a section showing details of the construction of part of theapparatus of FIGURE 1.

Referring to FIGURE 1, a housing 4 is shown having a threaded opening 6at its lower end in which is inserted an air hose connector 8 having acentral passage 10 leading to the interior 12 of the housing 4. Withinpassage 10 there is provided a check valve comprising seat 14 and ball16, and a spider 18 permitting the passage of air through passage 10 butpreventing the entrance of ball 16 into the interior of the housing. Asimilar air hose connector 20 is threaded to the upper end of hous-3,504,663 Patented Apr. 7, 1970 ing 4, and a passage 22 communicateswith interior 12 of the housing. Connector 20 has no check valve.

A rod 24 which is slidable in a bushing 26 in the housing is provided atone end with a push button 28 held away from the housing by a helicalspring 30, and at its other end with a disc 32 to which is bonded anannulus of packing material 34. Packing 34 closes a large bleed passage36 leading from interior 12 to the exterior. When push button 28 isdepressed, air is allowed to escape quickly from the interior throughpassage 36. The provision of passage 36 and push button 28 hasparticular utility in blood pressure measurement since it facilitatesquick removal of the cuff after diastolic pressure has been determined.

The opposite side of housing 4 is closed off by a metal diaphragm 38which is clamped against annular packing 40 by a member 42. Member 42 isprovided with a plurality of passages 44 leading to the exterior, and athreaded passage 46 in which is threaded an adjusting screw 48. Withinscrew 48 and along the axis thereof there is fastened a cylindrical pin50 which extends from the end of screw 48 and part way through acircular aperture 52 provided in the center of diaphragm 38.

It will be apparent that diaphragm 38 will be normally flat when thepressure on both sides is the same, but it is shown here deformed by apressure differential for the purpose of illustration.

Typically, connector 8 is connected to a pressure source such as asqueeze-bulb 8A by a line 8B, and connector 20 is connected to a hose20A leading to a pressure cuff 20B of the type used for determiningblood pressure. The part of the interior of the housing to the right ofdiaphragm 38 as viewed in the drawing, passage 22, hose 20A and pressurecuff 20B constitute on enclosure.

The theory of operation will become clear from reference to FIGURE 2,which shows in detail the relationship between diaphragm 38 andcylindrical pin 50. An annular passage 54 is formed between diaphragm 38and pin 50, and its length L is variable according to the degree ofdeformation of diaphragm 38 and the adjustment of the position of thepin by adjusting screw 48.

For any gas, the velocity of flow of the gas through an orifice havingan annular cross section of constant area can be expressed to a goodapproximation by:

V is the velocity,

P is the pressure difference, L is the length of the passage, K is aconstant.

It will be apparent that diaphragm 38 will be normally 38 will beproportional to the difference of pressure on either side of thediaphagm, and therefore length L is proportional to the pressuredifference P. It follows that the velocity of the air flowing throughpassage 54 is constant. Thus, the rate of change in pressure in thechamber to which passage 22 leads is also constant.

In order to realize the further advantage of the present inventiondiscussed above, it is necessary that adjusting screw 48 be adjusted sothat when the pressure differential is just below the range of pressuresof interest and above zero, the diaphragm should be in such a positionthat it just clears the end of pin 50 so that the length L of passage 54is zero. When the pin clears aperture 52, air will rush out throughpassage 52 from the interior 12 of the housing, and will remove any lintor dust that might have accumulated. When the invention is used inconjunction with a blood pressure measuring apparatus, adjusting screw48 might be adjusted so that aperture 52 clears pin 50 when the pressuredecreases to a value below the lowest expected diastolic pressure.

It will be apparent that pin 50 and aperture 52 should he of uniformcross-sectional area so that the cross-sectional area of passage 54 isconstant in order to obtain a constant velocity of flow through passage54. It is not necessary to use a tapered pin, and undesirable clogging,which is an accompanying characteristic of valves involving tapered pinsand tapered openings is avoided. An arrangement involving a tapered pinand a tapered hole would ordinarily involve the simultaneous variationof passage length and cross-sectional area. Since the velocity of flowvaries with diameter as well as with length, it would consequently bedifiicult to produce a constant air flow velocity without a morecomplicated arrangement of parts.

It will be apparent that the invention is not limited to bleed devicesfor use in conjunction with blood pressure measuring apparatuses, andthat numerous other uses may be made of the apparatus in accordance withthe present invention wherever it is desired to produce a constant rateof pressure drop in a fluid.

What is claimed is:

1. An apparatus for producing a constant rate of decrease of thepressure of a gas within an enclosure comprising an enclosure, aflexible diaphragm forming part of the boundary of said enclosure andmovable in re sponse to pressure changes within said enclosure, acylindrical aperture in said diaphragm, a cylindrical pin having asmaller cross-section than said aperature, means fastening saidcylindrical pin to a part of said enclosure to which the aperture isrelatively movable and holding said pin with its axis in parallelrelation to the axis of said cylindrical aperture and so that it extendspart way through said aperture from the side of the diaphragm outsidethe enclosure to form a passage having a substantially constantcross-section between the pin and the wall of said aperture when thepressure in said enclosure is at least greater by a predetermined amountthan the pressure on the opposite sides of said diaphragm.

2. A sphygmomanometer comprising an inflatable cuff, means communicatingwith said cuff, indicating means connected to the interior of said cufffor indicating the pressure of gas within said cuff, a chamber, apassage providing communication between said chamber and said cuff, aflexible diaphragm forming part of the boundary of said chamber andmovable in response to pressure changes within said cuff, a cylindricalaperture in said diaphragm, a cylindrical in having a smallercross-section than said aperture, means fastening said cylindrical pinto a part of said chamber to which the aperture is relatively movableand holding said pin with its axis in parallel relation to the axis ofsaid cylindrical aperture and .4 so that it extends part way throughsaid aperture from the side of the diaphragm outside said chamber toform a passage having a substantially constant cross-section between thepin and the wall of said aperture when the ressure in said chamber is atleast greater by a predetermined amount than the pressure on theopposite side of said diaphragm.

3. A sphygmomanometer according to claim 2 including a second passageproviding communication between the interior and exterior of saidchamber, and controllable valve means normally closing said secondpassage for quickly releasing gas from said chamber through said secondpassage when said valve means is opened.

4. A sphymomanometer in accordance with claim 2 including meansconnecting said means for supplying a gas under pressure to the cuflthrough said chamber, and a check valve at the inlet to said chamberfrom said means for supplying a gas to prevent deflation of the culithrough the gas supply means.

5. A sphygmomanometer comprising an inflatable cuflF, meanscommunicating with said cufi' for supplying a gas under pressure to thecuff, indicating means connected to the interior of the cult forindicating the pressure of gas within the cult, a chamber, a passageproviding communication between said chamber and said cutf, meansdefining a cylindrical aperture providing communication between theinterior and exterior of said chamber, a cylindrical pin having asmaller cross-sectional area than said aperture, means mounting said pinwith respect to said aperture so that the axes of said pin and apertureare parallel and so that said pin can extend into said aperture to forma restricted passage, having a substantially constant transversecross-section, between the pin and the wall of said aperture, and meansresponsive to the difference between the pressure inside said chamberand the pressure outside said chamber for producing relative movementbetween said pin and said aperture to decrease the length of saidrestricted passage linearly with a decrease in the pressure differencebetween the inside and outside of said chamber.

References Cited UNITED STATES PATENTS 3,105,477 10/1963 Lowther 138-45XR 3,164,141 1/1965 Jones 13845 XR RICHARD A. GAUDET, Primary ExaminerK. L. HOWELL, Assistant Examiner US. Cl. X.R. 138-46

